Add prebuilts/ to microfactory includes

This allows go sources under subdirectories of prebuilts/ to be
referencable by soong_ui and soong_build. Without this change,
microfactory does not correctly include go libraries under prebuilts.

Test: Conjunction with aosp/2260126
Change-Id: I4cba6fb04d1e274af759fa92f9ae4b2e3ea18447